Receivers employ a mixer to translate a higher input radio frequency (RF) to a lower baseband or intermediate frequency. This process is known as down-conversion when utilizing a difference frequency between the RF input signal and a local oscillator employed by the mixer for conversion. Ideally, the mixer output signal amplitude and phase are proportional only to the RF input signal amplitude and phase and independent of local oscillator characteristics. However, mixer nonlinearities produce undesirable mixing products.
A second-order nonlinearity associated with the mixer produces an undesirable intermodulation product between two RF input frequencies. Ideally, for example, two RF input signals having different frequencies that are provided to the mixer within its frequency band should not produce a cross-modulation signal since the ideal transfer characteristic of the mixer would be perfectly linear. However, non-ideal mixers have a transfer characteristic that is nonlinear to some degree corresponding to second-order and greater nonlinear terms. The second-order nonlinearity will cause a spurious cross-modulation signal to be generated from two in-band RF input signal frequencies applied to the mixer. If this spurious cross-modulation signal is too large, the performance of the mixer and the associated quality of reception are degraded.
The second-order nonlinearity of the mixer is related to mixer imbalance wherein several mixer attributes typically contribute to this imbalance. Ideally, devices used in the mixer should have identical or perfectly matched characteristics related to switching parameters or properties associated with conducting and not conducting states. Additionally, phase errors associated with multiphase mixers may also contribute to mixer imbalance, as well. That is, if two local oscillator phases employed in the mixer differ from their intended phases, mixer imbalance will typically result. Generally, each of these characteristics contributes in some degree to increasing the mixer imbalance and its corresponding second-order nonlinearity. Therefore, mismatch contributions from such a broad-based combination of characteristics essentially assures some degree of imbalance in a mixer.
Accordingly, what is needed in the art is an effective way to decrease the second-order nonlinearity of a mixer.